In satellite and microwave communications, often the same frequency band is utilized in several nonoverlapping spatial regions. In terrestrial systems, for example, each transmitter covers a specific "field of view" and receivers associated with the transmitter are disposed within the field of view and tuned to the proper frequency. For example, a first transmitter may cover a first portion of the United States and use the same frequency band as a second transmitter aimed toward a second portion of the United States. Depending on which field of view a receiver is located in, it will receive the transmissions of one of the transmitters.
Although a receiver in one field of view should be immune from communications in another, this is not the case in practical systems. Excess transmitter power, poorly aimed antennas, and scattering, are just three phenomena which may cause energy to "spillover" into a different field of view. This spillover causes interference in the receivers associated with other transmitters. In satellite communications, where several satellite channels overlap spatially but use different frequency bands, an analogous problem exists among interfering channels.
FIG. 1 is a block diagram which depicts the above discussed problem in a terrestrial/satellite communications system. Transmiters 101-103 are intended to transmit information to receivers 109-113, 114-117, and 104-108, respectively. Additionally, satellite 118 is intended to transmit information to several other receivers (not shown). It can be seen, that if the distance between any two of transmitters 101-103 is not large enough, transmissions from a transmitter will be received by receivers which are associated with a different transmitter.
In practice, in order to determine which particular one of many possible sources is causing the interference, the transmitters must be turned off, one at a time, until turning off a particular transmitter results in substantial reduction in interference. The particular transmitter is then identified as that causing the problem, and appropriate adjustments to this transmitter can then be made.
The problem with the above method of determining which transmitter is the source of interference is that it requires disruptions in service as each transmitter must be shut down for a shot time. Further, when a receiver exhibits degraded performance, there exists no concise method of identifying the problem as an interference problem in the first place. The problem is further compounded by the fact that interference may be caused by the intermodulation products of two or more transmitters making it difficult to determine which combination of transmitters should be adjusted to reduce the interference. Finally, in a satellite system, each of the transmitters may be transmitting to different time zones, making coordination even more difficult.